Dimerization reactions photooxidation

While it is in the triplet state a molecule may undergo typical diradical reactions. This provides a plausible mechanism for radical-like reactions of substances that are largely diamagnetic. They are partly converted to the triplet state by light, or in the case of low lying triplet states, by heat. Probable examples of this mechanism are the photooxidation of rubrene and the photooxidation and dimerization of anthracene and higher members of the acene series.76... 

Dimerization can also be achieved using nickel peroxide " or through a photooxidation reaction. In these cases, 4-monosubstituted-5(4//)-oxazolones 293 are converted to the corresponding 4,4 -bis(oxazolones) 294 (Scheme 7.97 ... 

Among photooxidation products of the betaines 86 (R = Me, PhCH2, Ph, = H) are N-substituted maleimides (107). Evidence for the intermediate 108 in these reactions is provided by the observation that photooxidation of l-methyl-5-phenylpyridinium-3-olate (86 R = Me, R = Ph) gives the peroxy dimer 109 (11%). ... 

Processes of axial ligand photooxidation lead to the production of radicals which are consumed in subsequent fast thermal (dark) reactions. Such radicals can dimerize, as e.g. alkyl radicals released from excited Co (III) organometallics [122]... 

This binuclear photooxidative addition reaction is general for a number of halocarbons (Figure 3). While DCE and 1,2-dibromoethane react cleanly to give the dihalide metal dimers and ethylene, substrates such as bromobenzene or methylene chloride react through an alkyl or aryl intermediate. This intermediate reacts further to yield the dihalide d2-d2 metal complexes. 

Tetraphenylethylene cyclizes anodically to 9,10-diphenylphenanthrene analogously to its photooxidative cyclization. The attempted anodic cyclization of cis- or frans-stilbene to phenanthrene however failed due to electrophilic reaction of the intermediate radical cation with the solvent 37S Primary aromatic amines are oxidized to radical cations which, depending on the pH of the electrolyte couple to aminodiphenylamines (C-N coupling (84) in Eq. (172) ), yield benzidines (85) at low pH (C-C coupling) or dimerize to hydrazobenzene (86) (N-N coupling) which is subsequently oxidized to azobenzene (Eq. (172) ) 2 5,376,377)... 

The photochemical conversion of the thiopyrimidine dimers to the parent thio-pyrimidines is most readily followed by the regeneration of the spectra of the latter (Fig. 6), as for the ketopyrimidine dihydrodimers (Fig. 2). The reaction may also be followed electrochemically by measurements of the reduction wave of the regenerated parent pyrimidine. The behaviour of the thiopyrimidine dihydrodimers differs essentially from that of the ketopyrimidine dihydrodimers 140) in that, under aerobic conditions, the rates of conversion of the former are considerably more rapid and exhibit an oxygen effect (Fig. 7a, b). The corresponding quantum yields under aerobic conditions are given in Table VI. Under anaerobic conditions the rates of photooxidation of Dt 3 decrease considerably (Fig. 7a), whereas that of D4 is barely affected and is somewhat more complex (Fig. 7b). 

For the dimers Dt 3 the electron and proton acceptor in the oxidation reaction is oxygen, in the absence of which photooxidation is inhibited. It should be noted that the maximum concentration of dimer employed in these reactions was 0.5 x 10-4 M, as compared to the oxygen concentration in an aerated solution of 3 x 10 4 M, so that an excess of oxygen prevailed in all instances. 

The first step in XJVR-induced skin cancer is UVR-initiated DNA mutation, which causes the transformation of the normal cells to malignant cells. For UVR to initiate a biological reaction, it has to be absorbed by endogenous molecules (chromophores). UVB is absorbed directly by the DNA, and therefore can directly induce DNA mutation (224), in the form of thymine dimer formation (289). Some protein components may also ad as chromophores for UVB (224). UVA is absorbed by the reduced forms of the co-enzymes nicotinamide adenine dinucleotide (NADH) and nicotinamide adenine dinucleotide phosphate (NADPH), tryptophan, riboflavin, and melanin (224,290). UVA-induced DNA damage is believed to be mediated by oxygen reactive species that are released after the absorption of UVA by those endogenous chromophores and results in photooxidation of selected bases... 

It has been found that Cj contains a special chlorophyll-a48 49 very probably in a dimer form50,51, called chlorophyll-aj (Chl-aj), which is primarily photooxidized with the concomitant reduction of an as yet not unequivocally identified primary electron acceptor (very recently evidence has been presented for the characterization as an Fe-S-protein, see Ref.52. Similarly, the photoelectric generator Cu contains a special chlorophyll-a53, called chlorophyll-ajj (Chi-an), which is also oxidized during the primary photoact54,5S. The primary electron acceptor was found to be a special plastoquinone molecule which is reduced to the semiquinone form56,57. It is called X 32056. Thus, the reactions at Cj and Cn can be explicitly formulated ... 

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